1. Field of the Invention
The invention relates to a method for determining the type of a disk and, more particularly, to a method for determining the type of a disk in a blank area with no data of the disk.
2. Description of the Related Art
As the information age comes, to deal with large mount of digital information with different formats, optical storage mediums with the advantages of large storage capacity, fast accessing speed, easily to be carried, and convenient to be stored, are becoming a necessary means for storing data.
Digital versatile disks (DVD), a new kind of medium for storing optical information, are superior to compact disks (CD) in data density and the capacity. Therefore, DVD optical drives gradually replace conventional optical drives and become the mainstream products. However, since a lot of data are stored in the CDs in the past, the DVD optical drives have to be capable of reading the CDs. Furthermore, due to different use and different supporting companies, the DVDs can be divided into different specifications such as DVD-ROM, DVD-R, DVD-RW, DVD-RAM, DVD+R, and DVD+RW, while the CDs can also be divided into CD-ROM, CD-R, and CD-RW. Therefore when designing the DVD optical drives, compatibility needs to be considered to satisfy users' needs.
Although the same pickup head is used for reading the CDs and the DVDs in the DVD optical drives, many structural differences exist between the two kinds of disks. FIG. 1 is a vertical cross-sectional view of a CD. Please referring to FIG. 1, a CD 100 has a thickness of about 1.2 mm, and has a recording layer 120, a reflective layer 130, and a printing layer 140 formed in order on a surface of a pre-grooved substrate 110. FIG. 2 is a vertical cross-sectional view of a DVD. Please referring to FIG. 2, a DVD 200 includes a dummy substrate 250 with a thickness of 0.6 mm, and a data substrate with a thickness of 0.6 mm. The two substrates are adhered to each other by an adhering layer 240 to form an optical disk with a thickness of 1.2 mm. The data substrate has a pre-grooved substrate 210 as a base, and a recording layer 220 and a reflective layer 230 are formed thereon in order.
Because the structure of the two optical disks is different, a pickup head of the optical drive must emit laser beams of different power levels, and read data using different methods. Therefore before reading data, the optical drive must determine the disk type and employ a suitable reading method to correctly read data recorded thereon.
In a conventional method, determining the type of a disk placed in an optical drive is achieved by calculating the reflection time of the laser beam. First, the laser beam is emitted from a pickup head onto a data area of a disk. When the laser beam reaches the surface of the disk, it produces a reflective signal (note that many pits are formed on the surface of a reflective layer); while when the laser beam reaches the reflective layer of the disk, it also produces a reflective signal. A time difference between the two reflective signals is compared with a predetermined critical value to determine the disk type. As shown in FIG. 1 and FIG. 2, the substrate 110 of the CD 100 is thicker than the substrate 210 of the DVD 200, and therefore when the laser beam irradiates the CD 100, the time difference of the reflective signals is larger than that of the DVD 200. The optical drive therefore can determine whether the disk is a CD or a DVD.
However, in the conventional method for determining a disk type, the laser beam irradiates the data area of the disk where data is stored, while the pits formed by burning or etching are formed on the substrate of the disk in the data area. The pits make the reflected light scatter or diffract, and a depth difference between a land area and a pit area on the disk makes the time difference of the reflective signals produce errors. Therefore, the optical drive may misdetermine the disk type because of the errors to fail to read data stored in the disk correctly.